(1) Field of the Invention
This invention relates broadly to a nozzle for use with a powder-fed plasma torch.
More specifically, this invention relates to an improved high-efficiency nozzle for use with a powder-fed plasma torch, such as a plasma transferred arc torch.
Further, this invention relates to an improved method for feeding powder into the plasma plume of a plasma torch such as a plasma transferred arc torch.
(2) Description of the Prior Art
The plasma transferred arc process for depositing a flow of heat-fusible powdered material on a substrate or workpiece is well known. In such process, an electric arc within a torch strips electrons from a plasma-forming gas, such as argon or helium, thus ionizing the gas and placing it in an energy state higher than that in the gaseous state, resulting in a very high temperature plasma. Heat-fusible powdered materials, such as metals, metallic alloys, metallic oxides and other ceramic materials and carbides, are introduced into the high temperature plasma and are softened or melted therein while being accelerated to high velocities. These softened or melted high velocity particles are then projected or sprayed onto a substrate or workpiece to provide a high purity, high density, strongly bonded coating on the said substrate or workpiece. In this manner, through proper selection of the powdered material, a coating can be provided with properties not inherent in the substrate or workpiece, such as wear resistance or corrosion resistance. For example, cobalt-based alloy powders are used to hardface substrates (i.e., to provide the substrates with a wear-resistant surface).
Various methods and apparatus have heretofore been proposed for introducing the powdered material into the plasma, which methods and apparatus have shortcomings in one way or another.
U.S. Pat. No. 4,672,171 (1987) to Cusimano et al discloses a powder-fed plasma transferred arc torch having a replacable nozzle threaded into the exit end of the torch. Powder is fed through the nozzle into the plasma plume or column through a plurality of passageways inclined at an angle to the longitudinal axis of the nozzle and radially spaced about the central orifice or arc port, which passageways terminate at the flat face of the nozzle spaced some distance away from the said central orifice. Experience has shown that, with this system for feeding powder to the torch, the streams of powder exiting the passageways in the nozzle will, because of the relatively great distance they travel before reaching the plasma plume or column, expand in cross-section (i.e., diffuse, spread out or lose coherency), to such an extent as to result in a loss of ten percent of the powder under normal conditions, representing powder blown off the substrate or workpiece. On smaller substrates or workpieces, the percentage loss of powder will be much higher, because of a smaller target area. The more expensive the powder, the less economical is this arrangement of powder feed. U.S. Pat. No. 4,104,505 (1978) to Rayment et al shows a similar arrangement for feeding powder to plasma generated in a torch. Powder feed lines communicate with the flat exit end of the torch at some distance away from the central bore of the torch. U.S. Pat. No. Re. 31,018 (1982) to Harrington et al also shows a plasma spray gun in which powder is introduced into the plasma downstream of the exit end of the nozzle. In this patent, the direction of introduction of the powder is perpendicular to the longitudinal axis of the nozzle. It would be expected that some of the powder would be blown away by the plasma and wasted.
U.S. Pat. No. 3,839,618 (1974) to Muehlberger discloses a powder-fed plasma transferred arc apparatus and method wherein the nozzle downstream of the electrode has a constricting throat leading to a flared or diverging bore. Powder is fed into the throat in an upstream direction (relative to the direction of plasma flow) through passageways inclined at an acute angle to the longitudinal axis of the nozzle, toward the electrode, to increase the dwell time of the powder in the plasma thereby to increase the temperature of the powder before it is projected onto the substrate. It would seem that this arrangement could, under certain operating conditions, result in plugging of the throat and powder feed passageways.
U.S. Pat. No. 3,591,759 (1971) to Stand discloses a plasma spray device for depositing heat-fusible powdered material onto a substrate. The powder is introduced into the plasma flow midway between the entrance and exit ends of the torch nozzle through passageways inclined at an angle to the longitudinal axis of the nozzle, immediately downstream of an abrupt expansion in the nozzle inner diameter, in such manner that the powder is carried within the torch, toward the substrate, on the surface of the plasma rather than being injected into the body of the plasma. It would seem that this arrangement likewise could, under certain operating conditions, result in powder adhering to the wall of the bore of the nozzle. Downstream of the exit end of the torch, the plasma and powder carried on the surface thereof converge. The operation of this apparatus is predicated on laminar flow of the plasma upstream of the point of introduction of the powder, and an abrupt change to turbulent flow of the plasma at the point of introduction of the powder. It is said that the abrupt expansion of the nozzle causes a pressure drop drawing the powder into the nozzle and forcing the powder into a revolving path in the nozzle.
U.S. Pat. No. 3,304,402 (1967) to Thorpe discloses a powder-fed plasma spray gun, wherein the powder is fed into the nozzle perpendicularly to the direction of flow of the plasma and upstream of the exit end of the nozzle, so that the powder will travel a substantial distance through the nozzle before leaving the nozzle. Experience has shown that, with this type of powder feed, some of the powders will adhere to the bore of the nozzle. U.S. Pat. No. 3,387,110 (1968) to Wendler et al and U.S. Pat. No. 3,914,573 (1975) to Muehlberger show generally similar arrangements, the powder entering the plasma at an acute angle relative to the direction of flow of the plasma, upstream of the exit end of the nozzle.
Powder-fed plasma torches of general interest are disclosed in U.S. Pat. Nos. 3,803,380 (1974) to Ragaller, 4,125,754 (1978) to Wasserman et al and 4,739,146 (1988) to Lindland et al.